1. Field of the Invention
The invention relates to a portable device comprising an overload protection device for motor-operated tools. The portable device comprises a drive shaft (input shaft) connected to a drive motor and rotatably driven by it, a driven shaft (output shaft) for driving a cutting tool and arranged essentially perpendicularly to the drive shaft, as well as a drum supported on the output shaft and driven in rotation by a drive pinion. The invention further relates to an overload protection device for an electrically driven machine tool, such as an angle grinder or the like. The overload protection device is arranged within a drive train between a tool and an electric motor driving the tool, wherein the machine tool has a gearbox. The invention also relates to an electrically driven machine tool such as an angle grinder having such an overload protection device.
2. Description of the Related Art
In the prior art it is known to arrange a coupling between a drive shaft and an output shaft in order to enable transmission or interruption of transmission of a rotational movement from a motor to a tool. With such devices it is possible to control the rotational movement of the tool while maintaining a constant rotary speed of the motor. The coupling/decoupling is realized in a conventional way by deflection of a driving means that upon actuation, as needed, connects the input shaft to the output shaft so that the input shaft transmits its movement onto the output shaft.
Systems with positive couplings of this kind are known wherein a positive-engaging element forms the driving means that can be moved between a positive-engaging position, in which the two shafts are connected to one another, and a release position, in which the two shafts are separated from one another.
Moreover, couplings of this kind are known that are arranged within a system operating by friction. In this case, the first shaft has a first surface. The second shaft has a second surface that is positioned opposite the first surface. In this case, the driving means is constituted by the second surface that can be moved between a release position and a position in which it contacts the first surface. This contact ensures by means of friction a coupling action of the two surfaces which results in a connection of the two shafts and transmission of the movement. Such coupling systems have in common that an actuation, for example, by an operator, is required for coupling or decoupling. In the case of a strong overload of the output shaft as is the case when the tool is blocked, the operator cannot act fast enough in order to actuate such a coupling.
U.S. Pat. No. 4,669,590 discloses a coupling system that has a drum connected to the drive shaft. The driving means are formed by clamping jaws connected to the output shaft. The operator must actuate the device in order to effect that the clamping jaws are displaced for contacting the drum so that the drive elements and output element are coupled. In the opposite case, the operator must actuate the device in order to move the clamping jaws in the opposite direction so that the two shafts are separated.
Centrifugal couplings are also known in which the driving means is formed by an inertia body connected to the drive shaft which by the effect of rotation of this shaft and the centrifugal acceleration comes into contact with the output shaft. The coupling connection is thus automatically generated as soon as the rotary speed (rpm—revolutions per minute) of the drive shaft surpasses a certain limit. Such a system, however, does not operate automatically for decoupling, and blockage of the tool does not cause the two shafts to be decoupled.
It has been attempted to integrate torque limiters. For example, devices are known where the transmission of the movement upon surpassing a certain torque can be interrupted. The principle of such a device resides in that the torque limiter is automatically decoupled as soon as a torque limit has been reached. Such devices, for example, are realized by a frictional connection wherein, when a certain limit of the torque between the parts is surpassed, the torque transmission between the two parts is interrupted but the two parts still rub against one another. Such a device has the disadvantage that the entire additional energy is essentially dissipated in the form of heat and that an excessive wear of the friction parts results because they continue to operate as long as the device is under load, i.e., as long as the motor rotates and the torque limit is surpassed.
Another known device is designed such that the output shaft is driven by the drive shaft by means of mutual contact of two slanted surfaces. The slanted surfaces are configured to be complementary to one another and remain pressed against one another by means of a pressing (expanding) device that is adjusted to a certain limit. When surpassing a certain torque to be transmitted, the limit of the pressing device is surpassed. The two slanted surfaces are no longer in contact with one another but glide past one another. The transmission of the movement is no longer ensured. Such a device has the disadvantage that the energy is lost in the form of noise and/or heat. The device remains active as long as the torque is maintained; this causes a significant wear.
A further disadvantage that is common to the aforementioned torque limiters because of their properties resides in that decoupling is only temporary. As soon as the torque between the two shafts is reduced and drops below the value of the predetermined decoupling torque, the device engages again. The output shaft is again driven by the drive shaft. Moreover, the limiters are not very effective in the case of blockage. The devices usually are not very loadable and wear quickly.
U.S. Pat. No. 5,653,509 discloses a device in which radial inertia elements that are connected to the output shaft form the driving means. However, the movement of the inertia elements exclusively in a radial direction can lead to a blockage within their housing during the engagement phase as well as during the automatic decoupling phase that occurs upon blockage of the tool.
No portable device of the prior art has addressed the problem of decoupling of the output shaft when the two shafts are angularly positioned relative to one another, as it is the case, for example, in angle grinding machines (angle grinders). In this type of devices, the two shafts are positioned perpendicularly or at a right angle to one another.